Fossil of herbivorous crocodile discovered in Argentina
April 13, 2016
By Peter Moon | Agência FAPESP – A new species of prehistoric crocodile has been found in Argentina. It was small, lived 80 million years ago, and was not carnivorous. Unlike the modern crocodile, Llanosuchus tamaensis was probably omnivorous/herbivorous, as indicated by the characteristics of its fossilized teeth. Its name derives from llanos, the semi-arid plains of northern Argentina.
A description of the new crocodyliform was published in the journal Cretaceous Research. The work was led by paleontologist Lucas Fiorelli, affiliated with Argentina’s National Scientific & Technological Research Council (CONICET).
Geologist Giorgio Basilici, from the University of Campinas’s Geoscience Institute (IG-UNICAMP) in Brazil, studied the sedimentology of the deposits containing the fossil to understand the environment inhabited by L. tamaensis. The research project was supported by FAPESP.
Crocodiles, alligators and gharials offer a dim portrait of a glorious past. Crocodilians are now confined to the world’s riverbanks and wetlands, although a single marine species subsists in Australia. For almost 100 million years during the Jurassic and Cretaceous eras, however, the Crocodylomorpha superorder coexisted and competed for food with dinosaurs on land and with mosasaurs and pliosaurs in the oceans.
Among the hundreds of extinct species whose fossils have been identified, the notosuchian suborder is perhaps one of the most interesting. It evolved in Gondwana, the ancient supercontinent that broke up approximately 180 million years ago into the land masses now known as Africa, South America, Australia, Antarctica, the Indian subcontinent, and the Arabian Peninsula.
The notosuchians, or southern crocodiles, were exclusively terrestrial. They had elongated paws and erect limbs, and moved more like quadrupeds than their sprawling cousins.
There were two groups of notosuchians, Fiorelli explained. One comprised fierce hunters. “The animals in this group were much larger and totally carnivorous,” he said. The largest and most voracious was Baurusuchus, 3 m long and weighing 400 kg. It lived 90 million years ago in the environs of Bauru, São Paulo State, Brazil.
Alongside the large carnivorous dinosaurs, Baurusuchus was the top-tier predator in the Bauru Basin, a biome that contained rivers and lakes but was dry and hot. The Bauru Basin was found throughout southeastern Brazil in the Upper Cretaceous, between 90 and 80 million years ago.
The other group of notosuchians is considered more advanced on the basis of its feeding habits. “Why a group of carnivores becomes herbivorous is a complicated question,” Fiorelli said. In the case of notosuchians, the reasons are unknown, but clearly a common ancestor abandoned the carnivorous voracity characteristic of crocodilians to become an omnivore or partial herbivore. Its descendants spread through central and southern South America, from Bolivia to Argentina, evolving into the dozen species that have been identified to date. L. tamaensis is the most recent example.
The greatest diversity of advanced notosuchians was probably in São Paulo State, where seven omnivorous or partially herbivorous species have been discovered: Caipirasuchus paulistanus, C. montealtensis and Morrinhosuchus, all found in Monte Alto; Adamantinasuchus, in Adamantina; Caryonosuchus, in Presidente Prudente; Mariliasuchus, in Marília; and Armadillosuchus, in General Salgado.
Armadillosuchus has the differentiated teeth of an omnivore and a bizarre beauty; its bony shield-like body armor resembles that of an armadillo.
The advanced notosuchians had two other traits in common besides dentition. None of them was large. They ranged from midsize, up to 2 m long in the case of Armadillosuchus, to small; Llanosuchus, for example, was only 80 cm long – half the size of an iguana or tegu lizard.
Another peculiarity of the group is skull shape. These animals had a very short snout, almost like a beak or small shovel, which they probably used to dig holes for burrows. “Mariliasuchus is believed to have had burrowing habits,” Fiorelli said. The evidence is the existence of paleoburrows in the same rock strata in which Mariliasuchus fossils were found.
The discovery of a new species of advanced notosuchian in northwestern Argentina is paleoenvironmentally significant. “What we see of the Cretaceous in La Rioja Province, northwest Argentina, is very similar to what see in the Bauru Basin,” Fiorelli said.
Basilici’s analysis of paleosols in the Los Llanos Formation, where Llanosuchus was found, indicated that the climate there was semi-arid 80 million years ago, despite high levels of rainfall. “They reached about 700 mm per year, which is a lot,” he said. “That means there was vegetation in the area. The rainfall must have been concentrated in a single season, and for most of the year the weather was very warm and dry.”
Very warm – but how warm? There was a pronounced greenhouse effect during the Cretaceous, Basilici explained. “Temperatures will probably have exceeded 50°C much of the time, possibly even reaching 60°C during the warmest and driest part of the year,” he said.
His inference that the climate was semi-arid derives from several factors, including signs of evaporation retained by the paleosols in the area and an abundance of calcium carbonate (CaCO3) nodules. “All this is evidence of aridity,” he said. “The site will have had plant cover adapted to the arid conditions. We found marks of fossil roots with diameters of up to 40 cm and reaching as deep as 10 m below ground. They were very long because they had to go way down to find water stored in the subsoil. I’d say this area of Argentina 80 million years ago was practically a desert, with a few scattered trees and bushes and probably grass.”
The notosuchians were also well adapted to the arid and semi-arid climate in the later Cretaceous. No one knows whether this helped them survive the great mass extinction that wiped out the large dinosaurs, with the exception of birds, 65 million years ago. In any case, the notosuchians did not make it to the modern era. “The last known species disappeared in the Miocene,” Fiorelli said. The Miocene Epoch refers to a span of geologic time from 5.3 million to 23.0 million years ago.
The article “A new Late Cretaceous crocodyliform from the western margin of Gondwana (La Rioja Province, Argentina)” by Fiorelli et al., published in Cretaceous Research 60: 194-209, can be read at www.sciencedirect.com/science/article/pii/S0195667115301270.
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